When the flu hit, the media wrote stories about the flu and then
people to jumped on Google and start searching about the flu.
That caused
Google Flu to predict huge outbreaks of the flu which caused
the media to write even more stories about the flu, using the
Google Flu prediction engine as a source ... and so on.

Google also made another mistake with the tool. It only looked at
the year-earlier CDC data once. As the CDC started reporting
actual cases, Google didn't keep using the CDC data to check the
accuracy of its predictions.

Google says it has updated the tool and will now be using actual
CDC data all season long.